Classial "cho" (modern chuu) is a contraction of "chuun" which can mean "like", "what", "how", "when" and "why". But, here it means as/like. I don't see the "as long as" meaning in this particular couplet.

I also agree with Darush. It definitely means 'as long as' or perhaps better said in English ' while' . "as it is" implies the way it is which is obviously not the case here.
The second part of the couplet clarifies the meaning :

We must appreciate the mirror while it exists, not after it has fallen and shattered.

قدر آینه بدانیمچو هست/ نه در آن وقت که افتاد و شکست
Now I'm sure it means "while/as long as", because هست means (to exist) which is a temporary existence for the mirror before it breaks, whereas است means (to be) which refers to the unchanging and permanent substance of something , and also the second part mentions "time وقت", so" چو" must imply duration here

قدر آینه بدانیمچو هست/ نه در آن وقت که افتاد و شکست
Now I'm sure it means "while/as long as", because هست means (to exist) which is a temporary existence for the mirror before it breaks, whereas است means (to be) which refers to the unchanging and permanent substance of something , and also the second part mentions "time وقت", so" چو" must imply duration here

QP SaaHib, this is a different matter(mirror!) nothing to do with 'reflecting/telling the truth'.
(and again, confusing Farsi prepositions!)

Click to expand...

I am sorry aaqaa-ye-darush but I don't quite follow. My concern is not with any deep meaning of the lines but just superficial meaning. As far as I know "aa'iinah" means "mirror". What Farsi preposition am I confusing?